Let $A = (1,0)$ and $B = (5,4).$  Let $P$ be a point on the parabola $y^2 = 4x.$  Find the smallest possible value of $AP + BP.$
Answer: Note that $A$ is the focus of the parabola $y^2 = 4x,$ and the directrix is $x = -1.$  Then by definition of the parabola, the distance from $P$ to $A$ is equal to the distance from $P$ to the line $x = -1.$  Let $Q$ be the point on $x = -1$ closest to $P,$ and let $R$ be the point on $x = -1$ closest to $B.$

[asy]
unitsize(0.6 cm);

real upperparab (real x) {
  return (sqrt(4*x));
}

real lowerparab (real x) {
  return (-sqrt(4*x));
}

pair A, B, P, Q, R;

A = (1,0);
B = (5,4);
P = (1.5,upperparab(1.5));
Q = (-1,upperparab(1.5));
R = (-1,4);

draw(A--P--B);
draw(graph(upperparab,0,6));
draw(graph(lowerparab,0,6));
draw((-1,-5)--(-1,5),dashed);
draw(P--Q);
draw(B--R);
draw(B--Q);

dot("$A$", A, S);
dot("$B$", B, E);
dot("$P$", P, SE);
dot("$Q$", Q, W);
dot("$R$", R, W);
[/asy]

Then by the triangle inequality,
\[AP + BP = QP + BP \ge BQ.\]By the Pythagorean Theorem, $BQ = \sqrt{BR^2 + QR^2} \ge BR = 6.$

Equality occurs when $P$ coincides with the intersection of line segment $\overline{BR}$ with the parabola, so the minimum value of $AP + BP$ is $\boxed{6}.$